Method and a connector arrangement for connecting and disconnecting a generator to a circuit with an existing alternating current

ABSTRACT

A method and a connector arrangement for connecting and disconnecting an electrical generator, such as a prime mover driven alternator ( 10 ), to a circuit with an existing alternating current such as the mains electricity supply ( 20 ).

CROSS REFERENCE TO RELATED DOCUMENT

This application is a 371 of PCT/GB01/00840, filed on Feb. 27, 2001,which claims priority of United Kingdom Application No. 0006114.3, filedon Mar. 15, 2000.

FIELD OF THE INVENTION

The present invention relates to a method and a connector arrangementfor connecting and disconnecting an electrical generator such as a primemover driven alternator to a circuit with an existing alternatingcurrent such as the main electricity supply.

BACKGROUND OF THE INVENTION

A number of problems are encountered when connecting and disconnectingan electrical generator to a circuit carrying an existing alternatingcurrent such as the main electricity supply.

An electrical generator can be started in isolation until it reachessuitable power quality and the appropriate frequency and phase, and thenconnected to the main electricity supply. However, the provision of acontrol device to monitor the power quality of a generator and thenconnect the generator to the main electricity supply when theappropriate power quality has been achieved is expensive, increases thecomplexity of the device and is liable to error.

When disconnecting the electrical generator from the main electricitysupply, arcing is produced between the contacts of the switch used fordisconnection. This arcing wears out the contacts of the switchimpairing performance and necessitating regular replacement which isinconvenient and expensive. Arcing also disturbs the quality of the mainelectricity signal which is prohibited. Arcing can also cause damage toother components and the starting of fires and its prevention by forexample the provision of an inert gas atmosphere is expensive.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome one or more of theabove problems.

According to a first aspect of the present invention a method ofconnecting a prime mover driven electrical alternator having twoterminals to a circuit with an existing alternating current comprises

-   -   connecting the two terminals of a prime mover driven alternator        together through a suitably low impedance such that the prime        mover arranged to drive the alternator does not move        substantially and cannot make the alternator generate a current,    -   initialising the prime mover arranged to drive the alternator so        that it is in a suitable condition to drive the alternator at        the frequency of the alternating current in the circuit to which        it is to be connected and    -   connecting the terminals of the alternator to a circuit with an        existing alternating current to cause the alternator to start        movement of the prime mover for the prime mover to drive the        alternator to generate an alternating current with a suitable        power quality for the circuit thereafter.

A Stirling engine is preferably used as the prime mover as it can beinitialised by being heated by an external heat source and then remainin a state ready to be brought into operation extremely quickly.Starting the Stirling engine only requires a small amount of energysince the pistons, which are the moving part, are relatively light andcan be accelerated from rest very easily. The acceleration of thepistons from rest is continued in the normal operation of the engine, bylinear simple harmonic motion.

The starting energy is provided by applying the alternating current fromthe circuit to which the prime mover driven alternator is to beconnected, which is usually main power, to the stator of the alternatorthrough a suitable impedance to limit the inrush current and to bringthe alternator closely into phase with the main.

According to a second aspect of the present invention a method ofdisconnecting a prime mover driven electrical alternator from a circuitwith an existing alternating current comprises

-   -   connecting an impedance in parallel with the prime mover driven        electrical alternator, the impedance having a sufficiently low        impedance to require a current in excess of that which the        alternator is able to deliver to prevent the prime mover from        driving the alternator and thus stalling the alternator and    -   disconnecting the alternator from the circuit with an existing        alternating current.

By stalling the alternator before disconnecting it from the main, thearcing caused when it is disconnected is substantially reduced.

According to a third aspect of the present invention an apparatus forconnecting a prime mover driven alternator to a circuit with an existingalternating current comprises

-   -   an impedance switchable into and out of a parallel arrangement        with the alternator, the impedance being of sufficiently low        value that the prime mover cannot drive the alternator to        produce a current around the parallel impedance and a switch to        connect the prime mover driven alternator to the circuit with an        existing alternating current.

According to a fourth aspect of the present invention an apparatus fordisconnecting a prime mover driven alternator from a circuit with anexisting alternating current comprises

-   -   an impedance switchable into a parallel arrangement with the        alternator, the impedance being of sufficiently low value that        the prime mover cannot drive the alternator to produce a current        around the parallel impedance and a switch to disconnect the        prime mover driven alternator from the circuit with an existing        alternating current.

BRIEF DESCRIPTION OF THE DRAWINGS

An example illustrating the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 shows a connector arrangement for connecting a prime mover drivenalternator to a circuit with an existing alternating current,

FIGS. 2 to 7 show the effective state of that circuit at variousintervals,

FIG. 8 shows a control system for controlling switches in the connectorarrangement,

FIG. 9 shows the sequence of steps followed by the control system toconnect a prime mover driven alternator to a circuit with an existingalternating current and

FIG. 10 shows the sequence of steps followed by the control system todisconnect a prime mover driven alternator from a circuit with anexisting alternating current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The prime mover which drives the alternator 10 in the present exampleshown in FIG. 1 is a Stirling engine as is well known in the art, suchas the one described in WO 94/04878 for example. However, any othersuitable prime mover such as a gas engine, an internal combustion engineor a steam turbine for example would be suitable. Any suitablealternator may be used but in the present example a linear alternatorhas been found to work particularly well with the reciprocating Stirlingengine. In the present example shown in FIG. 1 the main electricitysupply 20 between neutral 21 and a live terminal 22 acts as the circuitwith an existing alternating current. The alternator has two terminals11, 12 one of which 11 is connected to neutral. The other terminal 12 ofthe alternator 10 is connectable to the live main supply 22 by twoparallel paths 30, 40. The first electrical path 30 has a switch 31 anda meter 32 in series and the second electrical path 40 has a firstimpedance 41, which in the present example is 10 ohms, a secondimpedance 42, which in the present example is 27 ohms, a switch 43 and ameter 44 in series.

Between impedances 41 and 42 a line 50 connected to neutral 21 isprovided. The line 50 has two switches 51, 52 in series.

Before starting the Stirling engine and connecting the alternator to themain supply, switches 31 and 43 are opened and switches 51 and 52 areclosed to produce the effective circuit shown in FIG. 2 with resistor 41connected around the alternator 10. The impedance of resistor 41 isselected to be sufficiently low such that the prime mover drivenalternator 10 cannot produce a current required to pass through theresistor 41 so that the engine is stalled and kept in a stationarycondition. The engine (not shown), which in this case is a Stirlingengine, is then brought into a condition ready for use by applying heatto one end of its piston chamber and cooling the other end. When theengine is sufficiently “warmed up” to achieve steady state operation andit is desired to connect the alternator 10 to the main electricitysupply 20, switch 52 is opened and switch 43 is closed. Switches 52 and43 are provided by a “break before make” switching arrangement as iswell known in the art to ensure that switch 52 is open before switch 43is closed. The connector arrangement is then effectively in theconfiguration shown in FIG. 3 with the alternator 10 connected to themain 20 through the two impedances 41 and 42 in series. The particulartotal value of the impedances 41 and 42 is selected dependent upon theparticular prime mover and alternator combination that is used. In thepresent example, the sum of the series impedances has been selected at37 ohms which dictates the current which passes through the coils of thealternator and which in turn dictates the force applied to the movingportion of the alternator within the coils which in this case is apiston. The force must be sufficient to start movement of the piston andyet must not be so excessive that it produces so much force that thepiston damages the alternator or prime mover. As can be seen, theparticular impedance value selected would be dependent upon theparticular type of prime mover and alternator used. The connectorarrangement 1 of the present invention is particularly suitable for usewith a Stirling engine as the prime mover of the alternator as theStirling engine can be powered by an external heat source and thenremain in a steady state ready to be brought into operation extremelyquickly. Furthermore, starting the engine only requires a small amountof energy since the piston is relatively light and can be acceleratedfrom rest very easily. The acceleration of the piston from rest iscontinued in the normal operation of the engine by linear simpleharmonic motion.

Starting the prime mover driven alternator in this manner ensures thatthe alternator 10 produces electricity of a suitable power quality forthe main electricity supply with the same frequency and in phase.

The connector arrangement 1 of the present invention is provided with ameter 44 which provides an indication of the current passing throughline 40. After switch 43 is closed the current measured by meter 44 ischecked to ensure that it is within expected parameters. This checkprovides an indication that the impedances 41, 42 and contacts of theswitches 31, 43, 51, 52 are working satisfactorily. The current measuredby meter 44 is read by a controller 101 which compares it with expectedvalues to ensure that the connector arrangement 1 is functioningcorrectly. If the measured value is outside the desired values thecontrol means performs an emergency shut down procedure as describedlater. However, under normal circumstances, the reading of meter 44 willbe within expected parameters and so switch 31 is closed and switch 51is opened to produce the effective circuit shown in FIG. 4. Switches 31and 51 are provided in a “make before break” configuration as is wellknown in the art. After a suitable period of time has elapsed for thecurrent through line 30 to settle, for example 100 ms, the currentthrough meter 32 is checked to see whether it is within normal operatingparameters. If it is not then the connector arrangement undergoesemergency shut down procedures as are described later. However, undernormal circumstances the current will be within normal parameters andswitch 43 is then opened to eliminate the impedance provided byimpedances 41 and 42 such that the prime mover driven alternator isdirectly connected to the main as shown in FIG. 5. Since in the presentexample switches 43 and 52 are formed together in a “break before make”arrangement the opening of switch 43 also closes switch 52. However, nocurrent passes through line 50 because switch 51 remain open. The primemover driven alternator 10 continues to be directly connected to themain supply 20 as shown in FIG. 5 until it is desired to turn off theprime mover. During normal operation the current passing through meter32 is monitored at regular intervals to ensure that the connectorarrangement 1 is working satisfactorily and to ensure that the primemover driven alternator is operating within its normal specification.

When it is desired to disconnect the prime mover driven alternator 10from the main supply 20 the heater of the Stirling engine which is usedin the present example is turned off and the remaining heat in theStirling engine is used up. When the Stirling engine drops below acritical temperature it starts drawing power from the main 20 ratherthan supplying it as is well known in Stirling engine operation. Atemperature detector 15 provided on the Stirling engine is connected toa controller 101 (see FIG. 8) and when this critical temperature isreached, which for the Stirling engine used in the present example is300° C., switch 51 is closed to produce the effective circuit shown inFIG. 6 for a few milliseconds as the “make before break” switcharrangement of switches 31 and 52 changes state. In this arrangement theengine stalls because it cannot produce sufficient current to be driventhrough the impedance 41. The value of impedance 41 is selected for theparticular type of prime mover driven alternator 10 used such that theprime mover driven alternator cannot produce sufficient current to bedriven through the resistor 41 so that the engine stalls.

After a suitable period of time for the prime mover driven alternator tostall, in this example 1.5 ms, switch 31 of the “make before break”switch arrangement of switches 31 and 51 is opened to disconnect theprime mover driven alternator from the main 20 to produce the effectivecircuit shown in FIG. 7. Substantially less or no arcing is produced asthe prime mover driven alternator is already stopped before it isdisconnected from the main supply 20.

When the prime mover driven alternator is disconnected from the main 20in an emergency situation, for example because the current detected atmeters 32 or 44 is outside normal parameters, then switch 51 is closedand switch 31 is opened without waiting for the remaining heat in theStirling engine to be used up. This should prevent the prime moverdriven alternator from suffering any damage due to operation outside itsnormal conditions.

The arrangement of the control system for controlling switches 31, 51,43, 52 in the manner described above is shown in FIG. 8.

The controller 101 which may be a microprocessor or a computer forexample has data links 110, 111 to meters 32 and 44 respectively and adata link 112 to temperature detector 15 on the Stirling engine. Thecontroller 101 also has actuating links 115, 116 to “make before break”switch arrangement 31, 51 and “break before make” switch arrangement 43,52 respectively.

The sequence of operations followed by the control system for connectingthe prime mover driven alternator 10 to the main electricity supply 20is shown in FIG. 9.

At step 201 the connector arrangement 1 is initially put into thecondition in which:

-   -   SWITCH 31 is OPEN    -   SWITCH 43 is OPEN    -   SWITCH 51 is CLOSED    -   SWITCH 52 is CLOSED

At step 202

-   -   SWITCH 52 is OPENED and    -   SWITCH 43 is CLOSED

At step 203

-   -   A reading is taken from meter 44 and if it is outside expected        conditions emergency shut down is activated at step 208. If the        reading is inside expected conditions the control means proceeds        to step 204.

At step 204

-   -   SWITCH 31 is CLOSED and    -   SWITCH 51 is OPENED

At step 205

-   -   A reading is taken from meter 32 and if it is outside expected        conditions emergency shut down is activated at step 208. If the        reading is within expected conditions the control means proceeds        to step 206.

At step 206

-   -   SWITCH 43 if OPENED and    -   SWITCH 52 is CLOSED

At step 207

-   -   Meter 32 is monitored and if a reading is taken which is outside        expected conditions emergency shut down is activated at step        208. Otherwise the system continues with this switch arrangement        until disconnection is required as shown in FIG. 9.

The sequence of operations followed by the control system fordisconnecting the prime mover driven alternator 10 from the mainelectricity supply is shown in FIG. 10.

Initially

-   -   SWITCH 31 will be CLOSED    -   SWITCH 43 will be OPEN    -   SWITCH 51 will be OPEN and    -   SWITCH 52 will be CLOSED

At step 301 the heater for the Stirling engine is turned off and thetemperature of the Stirling engine is monitored on line 112 until itreaches a critical temperature at which the engine begins drawing or isabout to draw power from the main. When this temperature is reached thecontrol means proceeds to step 302.

At step 302 “make before break” switch arrangement 31, 51 is activatedsuch that

-   -   SWITCH 51 is CLOSED and then    -   SWITCH 31 is OPENED

For emergency shut down the control system proceeds directly to step302.

A Stirling engine driven alternator 10 and connector arrangement 1 canbe transported with the connector arrangement 1 arranged with switches31 and 43 open and switches 51 and 52 closed to produce the effectivecircuit shown in FIGS. 2 and 7. In this condition the piston of theStirling engine is held substantially stationary so that no damage willbe caused to the Stirling engine by movement or vibration duringtransportation.

The use of mechanical switches to perform the shutdown sequence ratherthan an external timing device enables shutdown to be performed even inthe event of loss of the power supply. The connector arrangement 1 isable to provide both connection and disconnection from the main powersupply with the same components thus lowering the component count byproviding dual functionality.

The connector arrangement 1 provides both a reliable main supplyconnection and a safe instant disconnection in the event of loss of mainelectricity supply. The method relies on connection to a circuit with anexisting alternating current such as the main.

As well as using the full circuit design, part of the circuit may beused for a particular connection or disconnection application. Forexample the disconnection circuit could be used for a stand alonegenerator and the connection circuit could be used for grid independentstarting.

1. A method of disconnecting a prime mover driven alternator from acircuit which is carrying an existing alternating current comprisingconnecting an impedance in parallel with the prime mover drivenalternator, whilst the alternating current exists within the circuit,the impedance having a sufficiently low impedance value to require acurrent in excess of that which the alternator is able to deliver toprevent the prime mover from driving the alternator and thus stallingthe prime mover and disconnecting the alternator from the circuit withan existing alternating current.
 2. The method according to claim 1, inwhich the prime mover is a Stirling engine and the alternator is alinear alternator.
 3. The method according to claim 2, in which theStirling engine has a heater and the heater is turned off and heat inthe Stirling engine is used up before the impedance is connected inparallel with the alternator to stall the Stirling engine.
 4. The methodaccording to any of claim 1 in which the circuit which is carrying anexisting alternating current is the main electricity supply.
 5. Anapparatus for disconnecting a prime mover driven alternator from acircuit which is carrying an existing alternating current comprising: animpedance switchable into a parallel arrangement with the alternator,the impedance being of sufficiently low value that the prime movercannot drive the alternator to produce a current around the parallelimpedance; a switch to disconnect the alternator from the circuit withan existing alternating current; and a make-before-break switch toconnect the impedance into a parallel arrangement with the alternatorbefore disconnecting the alternator from the circuit which is carryingan existing alternating current.
 6. An apparatus for disconnecting aprime mover driven alternator from a circuit which is carrying anexisting alternating current comprising: an impedance switchable into aparallel arrangement with the alternator, the impedance being ofsufficiently low value that the prime mover cannot drive the alternatorto produce a current around the parallel impedance and a switch todisconnect the alternator from the circuit with an existing alternatingcurrent, wherein the prime mover driving the alternator is a Stirlingengine and wherein supply of heat to the Stirling engine is interruptedbefore the impedance is switched into a parallel arrangement with thealternator.